A. Field of the Invention
The present invention relates generally to cardiac stimulating devices. More particularly, the present invention relates to an implantable cardiac pacemaker or cardioverter/defibrillator.
B. Description of the Related Art
In the normal human heart, illustrated in FIG. 1, the sinus (or sinoatrial (SA)) node generally located near the junction of the superior vena cava and the right atrium constitutes the primary natural pacemaker by which rhythmic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers (or atria) at the right and left sides of the heart. In response to excitation from the SA node, the atria contract, pumping blood from those chambers into the respective ventricular chambers (or ventricles). The impulse is transmitted to the ventricles through the atrioventricular (AV) node, and via a conduction system comprising the bundle of His, or common bundle, the right and left bundle branches, and the Purkinje fibers. The transmitted impulse causes the ventricles to contract, the right ventricle pumping unoxygenated blood through the pulmonary artery to the lungs, and the left ventricle pumping oxygenated (arterial) blood through the aorta and the lesser arteries to the body. The right atrium receives the unoxygenated (venous) blood. The blood oxygenated by the lungs is carried via the pulmonary veins to the left atrium.
This action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. Four one-way valves, between the atrial and ventricular chambers in the right and left sides of the heart (the tricuspid valve and the mitral valve, respectively), and at the exits of the right and left ventricles (the pulmonic and aortic valves, respectively, not shown) prevent backflow of the blood as it moves through the heart and the circulatory system.
The sinus node is spontaneously rhythmic, and the cardiac rhythm it generates is termed normal sinus rhythm ("NSR") or simply sinus rhythm. This capacity to produce spontaneous cardiac impulses is called rhythmicity, or automaticity. Some other cardiac tissues possess rhythmicity and hence constitute secondary natural pacemakers, but the sinus node is the primary natural pacemaker because it spontaneously generates electrical pulses at a faster rate. The secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node.
Disruption of the natural pacemaking and propagation system as a result of aging or disease is commonly treated by artificial cardiac pacing, by which rhythmic electrical discharges are applied to the heart at a desired rate from an artificial pacemaker. If the body's natural pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, an implantable pacemaker often is required to properly stimulate the heart. An artificial pacemaker (or "pacer" as it is commonly labeled) is a medical device which delivers electrical pulses to an electrode that is implanted adjacent to or in the patient's heart in order to stimulate the heart so that it will contract and beat at a desired rate. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Pat. No. 4,830,006.
Pacers today are typically designed to operate using one of three different response methodologies, namely, asynchronous (fixed rate), inhibited (stimulus generated in the absence of a specified cardiac activity), or triggered (stimulus delivered in response to a specified hemodynamic parameter). Broadly speaking, the inhibited and triggered pacemakers may be grouped as "demand" type pacemakers, in which a pacing pulse is only generated when demanded by the heart. Furthermore, to determine what pacing rate is required by the pacemaker, rate-responsive demand pacemakers may sense various conditions such as heart rate, physical exertion, temperature, and the like. Moreover, pacemaker implementations range from the simple fixed rate, single chamber device that provides pacing with no sensing function, to highly complex models that provide fully automatic dual chamber pacing and sensing functions. The latter type of pacemaker is the latest in a progression toward physiologic pacing, that is, the mode of artificial pacing that most closely simulates natural pacing.
Because of the large number of options available for pacer operation, an industry convention has been established whereby specific pacer configurations are identified according to a code comprising three or four letters. A fifth coded position may be used to describe a pacemaker's ability to respond to abnormally high heart rates (referred to as tachycardia). Because most pacemakers do not provide any antitachycardia functions, the fifth coded position is not used in most commonly used pacemaker types. Thus, most common configuration codes comprise either three or four letters, as shown in Table I below. For this reason and for simplicity's sake, the fifth code position is omitted from the following table. Each code can be interpreted as follows:
TABLE I ______________________________________ Code position 1 2 3 4 ______________________________________ Function chamber chamber response to programmability, Identified paced sensed sensing rate modulation Options 0 - none 0 - none 0 - none 0 - none Available A - atrium A - atrium T - triggered P - programmable V - ventricle V - ventricle I - inhibited M - multi- D - dual D - dual D - dual programmable (A + V) (A + V) (T + I) C - communicat- ing R - rate modulat- ing ______________________________________
For example, a DDD pacer paces either chamber (atrium or ventricle) and senses in either chamber. Thus, a pacer in DDD mode, may pace the ventricle in response to electrical activity sensed in the atrium. A VVI pacer paces and senses in the ventricle, but its pacing is inhibited by spontaneous electrical activation of the ventricle (i.e., the ventricle paces itself naturally). In VVIR mode, ventricular pacing is similarly inhibited upon determining that the ventricle is naturally contracting. With the VVIR mode, the pacer's pacing rate, however, in the absence of naturally occurring pacing at an appropriate rate, is modulated by the physical activity level of the patient. Pacers commonly include accelerometers to provide an indication of the patient's level of physical activity.
As illustrated in the table above, it may be desired to sense in one cardiac chamber (i.e., detect electrical activity representative of contraction of the chamber and referred to as a "sensed event") and, in response, pace (referred to as a "paced event") in the same or a different chamber. In general, most pacemakers today incorporate a sensing function to detect electrical activity at the site of one or more electrodes. The sensing circuit in the pacemaker (often referred to as the "sense" circuit) receives the electrical signals from the electrodes and determines when a physiologically significant event has occurred. Accordingly, if the heart's natural pacemaker is able to make the heart beat properly, the pacemaker's sense circuit detects the naturally occurring electrical impulses and determines that the heart is beating properly on its own.
Most pacemaker sense circuits incorporate an amplifier that amplifies the electrical signals received from the electrodes. Sense circuits typically also incorporate, or are coupled to, a comparator circuit that compares the magnitude of the amplified signal received from an electrode to a reference signal. When the amplified signal from the electrode exceeds the amplitude of the reference signal, the pacemaker determines that a physiologically significant event has occurred. In this context, the physiologically significant events are cardiac events, such as a contracting heart chamber. It is important for a pacemaker to accurately determine when a cardiac event has occurred. That is, a pacemaker should detect a true cardiac event, but not respond to non-cardiac signals.
In early pacemakers, the thresholds of the sense circuit were set during manufacture. However, preset thresholds often resulted in inappropriate pacing therapy because the amplitude of the electrical events in the heart varies widely from one patient to another. Further, changes in the amplitude of the electrical signals are common in the same patient as a result of a variety of factors, such as encapsulation of the electrode by fibrotic tissue, movement of the lead, changes and deterioration of the lead and other lead-related issues. In addition, the amplitude of the cardiac signal will vary due to changes in the electrophysiology of the heart. This latter effect is most drastic at the onset and progression of tachycardia (abnormally fast heart rate) and fibrillation (complete lack of blood pumping capacity), which are accompanied by a relatively rapid and sustained change in the amplitude of cardiac electrical events. For bradycardia (excessively slow rate) applications, large variations in a portion of the cardiac signal commonly referred to as the "P-wave" may occur as a result of patient movement and respiration, particularly when the patient's atrial electrodes are not anchored unto the heart wall, but "float" in a chamber.
In order to cope with these amplitude variations in the cardiac signal, some implantable cardiac stimulators include threshold circuits that are programmable by an attending physician. Such devices normally store information regarding the amplitude of the cardiac electrical signals in memory incorporated within the implant. While a patient is at a medical facility, a physician is able to establish a communication link to the implanted device with the aid of external programmer. The amplitude information stored previously by the implanted device is then transmitted to the external programmer. The physician analyzes this data and reprograms the implanted device's sense circuit to a suitable sensing threshold.
Other implanted devices include "automatic gain control" ("AGC") in which the implanted device is itself able to determine and select a suitable threshold setting without requiring the assistance of an external programmer and attending physician. Various AGC methods have been suggested and generally are useful for coping with the fast changing cardiac signal amplitudes characteristic of certain diseases and conditions. Although AGC methods attempt to track the cardiac signals and adapt the sense thresholds automatically in an optimal manner, limiting their operating range is nevertheless required to ensure that noise, artifacts or other electrical signals are not detected as electrical events originating from the heart chamber to which the sense circuit is associated. The limits imposed on present day AGC methods are determined based on the observed amplitudes of the cardiac signals. Thus, whether or not a cardiac stimulator employs AGC, it is desirable to provide to a physician information regarding the observed cardiac signal amplitudes.
Until now, implantable cardiac stimulators have included dedicated circuitry to measure and track the cardiac signal amplitude. Such circuitry is usually quite complex, consumes battery power, and depletes the limited space inside the implanted device. Because implantable cardiac stimulators normally are powered by a limited-life battery, it is desirable for the implant to consume as little power as possible. Further, the device should be as small and reliable as possible. With these design goals in mind, it is always desirable for an implantable medical device to include as few components as possible to minimize the number of components that can fail, thereby increasing reliability. Further, an implant with fewer components will generally consume less electrical power.
Accordingly, there is a need for an implantable cardiac stimulator that provides cardiac signal amplitude information to an external programmer using simpler and more reliable circuitry. Such a device would preferably include fewer circuit components compared to prior art devices. Despite the advantages such a device would offer, to date no such device is known to exist.